The present invention relates to image sensors, and more particularly to integrated CMOS image sensors with dark voltage correction.
In a CMOS imager, an image is focused upon an array of active pixels, where each active pixel provides an analog voltage signal indicative of a time integration of the intensity of light impinging upon a pixel diode. The time integration is determined by the frame rate, where the entire pixel array is read during each frame. Competing factors in choosing a frame rate are that a frame rate must be chosen fast enough so that images are reasonably clear (due to subject motion) but slow enough so that there are a sufficient number of photons to be detected by the pixel arrays. The analog voltages provided by the CMOS imager are processed and eventually digitized for use by computer systems, digital storage devices, or perhaps processed further by digital processing methods.
As frame rates increase, there is more of a burden placed upon signal processing circuits, and the analog-to-digital conversion performed to digitize the image may result in a bottleneck. This bottleneck can be reduced by utilizing a plurality of analog-to-digital converters (ADC). For example, an ADC can be dedicated to each column of pixels within the pixel array. However, to increase reliability, reduce manufacturing costs, and decrease power consumption, it is desirable to integrate the image sensor and its accompanying signal processing circuitry, including any ADCs, onto a single die. Employing a large plurality of ADCs would therefore make full integration more difficult. Therefore, it would be advantageous to structure the analog signal processing in such a way that only one or very few ADCs are needed to support high frame rates.
CMOS image sensors are affected by dark voltage. Dark voltage in a CMOS pixel results from dark current in the pixel diode caused by the leakage current in the pixel diode. Dark current increases with temperature, and hence the term dark voltage. Dark current acts like photon induced current and creates an error voltage in the pixel which is referred to as the dark voltage. Unless corrected, dark voltage causes the final image processed from the imager to appear with a brighter background than it should have. Dark current phenomenon not only increases the floor noise, it also hides darker points in the images.
To reduce the effects of dark voltage, prior art image sensors use a mechanical shutter to read the pixel array voltages when the shutter is closed, thus providing an array of dark voltages which are subtracted from the pixel array voltages when the shutter is opened to capture an image. This method requires substantial storage to store the array of dark voltages, and also reduces the effective frame rate because image frames are lost when the shutter is closed. Therefore, it would be advantageous to compensate for the dark voltages within the pixel array voltages without needing extra storage and without reducing the effective frame rate.